The present invention relates generally to measurement systems, and, more particularly, to a system and method for measuring water currents in real time.
Offshore drilling operations often involve a multitude of subsea tools, equipment, and operating procedures. One of the most significant operations is the deployment, monitoring, and servicing of the drilling riser and blow out preventer (BOP). The drilling riser is the conduit for returning drilling fluids and cuttings from the well to the platform or vessel above the water surface and for conveying well gas that may need to be diverted in well control operations. The drill string extends through the drilling riser and BOP. The drilling riser is composed of a number of sections of large diameter steel tubes joined with special connectors and can be a half mile or more in length (in deepwater developments). The riser may also support kill and choke lines, a mud booster, and other ancillary lines that connect a marine drilling vessel to the undersea wellhead. The riser is usually tensioned at the top and connected to the drilling vessel by way of a telescoping slip joint. The drilling riser, however, is not tied to supporting framework such as the conductor guides of traditional bottom-founded platforms. The telescoping slip joint permits relative vertical movement of the drilling vessel versus the vertically stationary riser. Horizontal movement is, to a small extent, tolerated and allowed for by means of ball or flex joints at the top of the BOP, and at the top of the telescoping joint. However, horizontal movement must be limited to avoid damage to the riser and the associated and enclosed equipment.
Offshore drilling elements such as drilling and production risers are under the influence of ocean currents and are subject to drag forces and vortex induced vibration (VIV). Floatation modules such as buoyant air cans or syntactic foam modules may be deployed along the length of the drilling riser to render it neutrally buoyant, although forces felt by the drilling riser as a result of horizontal or lateral loading from water currents are not alleviated by the addition of floatation modules. To the contrary, the presence of floatation modules around the circumference of the drilling riser materially increases the profile presented by the riser to the water current and contributes to greater drag and VIV effects on the drilling riser. Substantial water drag and VIV induced by currents present a danger to a riser of any great length. VIV causes the riser pipe to shake, which leads to fatigue and which may eventually lead to deterioration of the steel in the pipe. In addition, a drilling riser is also affected and stressed by its own weight, its top tension, the weight of drilling fluid, and wave and current action in the water.
In high current environments, VIV can lead to the premature failure of equipment, requiring the temporary or extended halt to the drilling operation because of the equipment failure. To prevent damage, drilling operations may have to be curtailed and the riser disconnected when wave and current conditions are excessive. Further, lateral load from drag may deform the drilling riser to a bowed shape that presents excessive angles with respect to the derrick at the top and the well at the bottom. As a result, as the drill string rotates within the drilling riser, the drill string may contact the drilling riser at these transition points and be subjected as a result to excessive and premature wear.
Other reasons for disconnecting the drilling riser include anticipated heavy weather (rig motions become too Large in response to high winds and seas) or the inability of the vessel to remain on location over the well site (due to heavy weather, equipment malfunctions, and operator error). If sufficient time is available, the drill string is withdrawn to the rig before disconnecting the drilling riser. If the drilling string is not withdrawn to the rig before disconnecting the riser, drilling equipment can be seriously damaged and in some cases a serious accident or oil spill could result. Conversely, if an emergency disconnect is made prematurely in a situation in which the predicted displacement limit would not have been reached by actual conditions, the resulting monetary expenditure due to possible mud loss and the cost of the time required to reconnect (possibly involving the retrieval of the sheared portion of drill string from within the well) and recommence drilling operations could easily be in the order of hundreds of thousands of dollars.
Because of the dangers posed by drag and VIV, ocean currents beneath and around offshore rigs are typically measured and monitored, often by using a current profiler such as an acoustic Doppler current profiler (ADCP). The measurement of accurate current velocities is important in such fields as weather prediction, biological studies of nutrients, environmental studies of sewage dispersion, and commercial exploration for natural resources, including crude oil. An ADCP is a current-measuring instrument that transmits through the water high frequency acoustic signals, which when downconverted to human hearing frequencies sound like xe2x80x9cpings.xe2x80x9d The current is determined by a Doppler shift in the backscatter echo from plankton, suspended sediment, and bubbles of the water, all assumed to be moving with the mean speed of the water. Time gating circuitry is employed which uses differences in acoustic travel time to sort the currents measured into range intervals, called depth cells or bins. The allocation of measured currents into depth cells permits the development of a three-dimensional profile of the speed and direction of current over part or all of a water column. An ADCP can be deployed from a vessel, a buoy, or a bottom platform. Typically, ADCPs are used to measure horizontal current velocities in a vertical column. The vertical water column is divided into a number of depth cells along the length or height of the water column. Dividing the data of the ADCP in this manner produces a profile of water velocities along the height of the water column.
Data gathering of oceanographic parameters has increased in recent years. Known techniques for measuring water currents have used moored current meters such as ACDPs to gather data representative of currents. In addition to the use of ADCPs for the measurement of currents, other meters are moored to measure temperature and other ocean characteristics. Time series recording of data of this sort is generated from fixed instruments on taut-wire moorings in the deep ocean and on continental shelf and slope locations.
ADCP meters that are moored to a drilling rig are rarely stationed more than a few hundred meters from the water surface. Without a deep moored ADCP, however, no data can be gathered concerning the speed and direction of current at greater depths. This lack of data leaves oceanographers unable to collect data concerning the currents that exist at depths that are not reached by a moored ADCP. Because a drilling riser may be buffeted in different directions at different depths, water currents at depth levels beneath the deepest of the moored ADCP meters are of significant concern to an oceanographer charged with predicting water currents along the length of the drilling riser. The often sparse and inadequate data generated by moored ADCPs has prompted efforts by scientific researchers to install a current meter on a remotely operated vehicle (ROV).
In this configuration, the current meter is fixed to the ROV, which travels in a downward direction adjacent to the drilling riser. During the travel, the current meter records current measurements as it travels along the length of the drilling riser. This measurement technique is deficient in that the presence of the ROV substantially distorts the measurements made by the current meter.
In accordance with the present invention, a method for measuring water currents is provided that substantially eliminates or reduces the disadvantages and problems associated with existing methods for measuring water currents. The measuring system of the present invention allows for the measurements of currents in real time through the entire depth of a water column, such as a deep sea water column that is adjacent to a drilling and/or production riser. The system comprises an ROV, an ADCP, and a computer system. The current profiler or ADCP is coupled to the ROV. As the ROV moves vertically within the water column, data collected by the ADCP and ROV is transmitted to a computer system located at a platform or vessel. The computer system receives the data, and processes and outputs real-time data concerning the velocity and heading of currents in the water column. The current profiler or ADCP may include a shroud to shield the transducers of the ADCP from extraneous noise and to dampen mechanical vibrations from the ROV.
One advantage of the present invention is the ability to receive data indicative of the current velocity, current heading, and depth in real-time. This data can then be processed in real-time to create a nearly instantaneous water current profile of the entire water column. Real-time profiling is especially advantageous with respect to deepwater drilling, because deepwater drilling risers are fastened at the water surface and at the seabed, but not tied to supporting framework. Potentially dangerous horizontal movement of the drilling vessel and/or risers may be closely monitored in real time, in accordance with the present invention. Thus, real-time profiling provides data that are crucial to limiting or preventing equipment damage caused by water currents or VIV that may be made more dangerous with the addition of floatation modules to the riser.
Another advantage of the present invention is the ability to enable prediction of drilling vessel and riser movement and stress. Drift of the drilling vessel, heavy or otherwise dangerous currents, and VIV may be predicted on the basis of data provided by the present invention. Successful prediction, in turn, is advantageous in that equipment damage and oil or gas leakage may be averted, saving downtime, repair, and cleanup expenses. Premature and costly disconnection of the riser may also be averted if currents and VIV are predicted to be manageable.
In profiling an entire water column, the present invention is further advantageous in that it automatically removes bad data points through processing. Bad data points may include points outside the water column or on the seabed. Automatically removing spurious data points from the calculation is necessary for the integrity of real-time data processing. A further advantage of the present invention concerns the prevention of false or inaccurate data points that are caused by noise or vibration. The shroud covering the top and sides of the current profiler shields the current profiler from noise and vibration generated by the ROV and the environment. Another advantage of the present invention is that the processing of the data may be conducted only on the basis of data collected by the ADCP, if it is determined that the data collected by the ADCP is of a sufficient quality. Other advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.